1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, an in-plane switching mode liquid crystal display device.
2. Discussion of the Related Art
Twisted nematic liquid crystal display devices (hereinafter TN LCDs) having high image quality and low power consumption are widely applied to flat panel display devices. TN LCDs, however, have a narrow viewing angle due to refractive anisotropy of liquid crystal molecules. This is because horizontally aligned liquid crystal molecules prior to applying a voltage become nearly vertically aligned with respect to the substrate when voltage is applied to the liquid crystal panel.
To solve this problem, a multi-domain LCD such as a two-domain TN LCD (TDTN LCD) and a domain divided TN LCD (DDTN LCD), and a TN LCD including an optical compensation film have been introduced. In these LCDs, however, a contrast ratio is decreased and a color shift is generated according to a viewing angle.
Further, for the purpose of obtaining a wide viewing angle, an in-plane switching mode LCD has also been proposed.
FIG. 1A is a plan view of a unit pixel of a conventional in-plane switching mode active matrix LCD. As shown in the drawing, the apparatus comprises a data bus line 1 and a gate bus line 2 which are arranged perpendicularly and/or horizontally in a matrix on a transparent substrate thereby defining a unit pixel region. A common line 5 is arranged parallel to the gate bus line 2 in the pixel region. A thin film transistor (TFT) is formed adjacent a cross point of the data bus line 1 and the gate bus line 2. A common electrode 11 and a data electrode 19 are formed in the pixel region.
FIG. 1B is a sectional view according to line I-I of FIG. 1A. As shown in the drawing, the TFT includes a gate electrode 10 electrically coupled with the gate bus line 2, a gate insulator 10 on a gate electrode 10, an a-Si layer 15 on a gate insulator 13, an n+a-Si layer 16 on the a-Si layer 15, and source/drain electrodes 17, 18 which are electrically coupled the data bus line 1 and the data electrode 19, respectively. The gate insulator 13 is formed on the inner surface of a first substrate 4. The common electrode 11 is formed with the gate electrode 10 and electrically coupled to the common line 5. Further, a passivation layer 20 and a first alignment layer (21) are deposited on the inner surface of the first substrate 4.
On a second substrate 6, a black matrix 7 is formed to prevent a light leakage around the TFT, the data bus line 1, an the gate bus line 2. A color filter later 8, an over-coat layer 9, and a second alignment layer (22) are formed on the black matrix 7 in sequence. Finally, a liquid crystal layer is formed between the first and second alignment layers.
The above conventional in-plane switching mode LCD, however, has a complex pattern in the pixel region thereby decreasing aperture ratio. The following formula (1) represents a light transmittance (T/To) in the liquid crystal layer of the conventional in-plane switching mode LCD.T/To=sin2(2α)sin2(πdΔn/λ)  (1)
, where α is an average rotating angle of liquid crystal directors, dΔn is a retardation by the liquid crystal layer, d is a thickness of the liquid crystal layer, Δn is the refractive anisotropy of the liquid crystal layer, and λ is a wavelength of light.
As shown in the above formula, if each of liquid crystal molecules having fixed conditions α, and Δn is used, a desirable light transmittance can be achieved in accordance with the thickness d of the liquid crystal layer.
The Japanese Patent Unexamined Publication No. 6-160878 proposed a TFT-LCD having a high light transmittance by providing a high aperture ratio and preventing a degradation failure in the pixel region. Further, the LCD may have been manufactured rapidly by choosing freely a liquid crystal compound and an alignment material. Furthermore, in this LCD, the increased light transmittance may have been achieved by the rotation of the liquid crystal molecules and by permitting dΔn to be in a range of 0.21-0.36 um.
In general, since a color-display in the LCD is based on the transmittance variation of the wavelength according to dΔn, it is essential to determine a desire dΔn so as to display all colors.
However, JP No. 6-160878 only discloses dΔn considering the light transmittance in the liquid crystal layer. The conventional TFT-LCD including JP No. 6-160878 does not disclose a desirable dΔn that considers the effect of color-shift.